// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build !amd64,!ppc64,!ppc64le
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package cipher
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import (
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"runtime"
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"unsafe"
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)
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// xorBytes xors the bytes in a and b. The destination should have enough
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// space, otherwise xorBytes will panic. Returns the number of bytes xor'd.
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func xorBytes(dst, a, b []byte) int {
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n := len(a)
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if len(b) < n {
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n = len(b)
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}
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if n == 0 {
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return 0
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}
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switch {
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case supportsUnaligned:
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fastXORBytes(dst, a, b, n)
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default:
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// TODO(hanwen): if (dst, a, b) have common alignment
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// we could still try fastXORBytes. It is not clear
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// how often this happens, and it's only worth it if
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// the block encryption itself is hardware
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// accelerated.
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safeXORBytes(dst, a, b, n)
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}
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return n
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}
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const wordSize = int(unsafe.Sizeof(uintptr(0)))
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const supportsUnaligned = runtime.GOARCH == "386" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOARCH == "s390x"
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// fastXORBytes xors in bulk. It only works on architectures that
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// support unaligned read/writes.
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// n needs to be smaller or equal than the length of a and b.
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func fastXORBytes(dst, a, b []byte, n int) {
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// Assert dst has enough space
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_ = dst[n-1]
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w := n / wordSize
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if w > 0 {
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dw := *(*[]uintptr)(unsafe.Pointer(&dst))
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aw := *(*[]uintptr)(unsafe.Pointer(&a))
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bw := *(*[]uintptr)(unsafe.Pointer(&b))
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for i := 0; i < w; i++ {
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dw[i] = aw[i] ^ bw[i]
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}
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}
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for i := (n - n%wordSize); i < n; i++ {
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dst[i] = a[i] ^ b[i]
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}
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}
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// n needs to be smaller or equal than the length of a and b.
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func safeXORBytes(dst, a, b []byte, n int) {
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for i := 0; i < n; i++ {
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dst[i] = a[i] ^ b[i]
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}
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}
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// fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
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// The arguments are assumed to be of equal length.
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func fastXORWords(dst, a, b []byte) {
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dw := *(*[]uintptr)(unsafe.Pointer(&dst))
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aw := *(*[]uintptr)(unsafe.Pointer(&a))
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bw := *(*[]uintptr)(unsafe.Pointer(&b))
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n := len(b) / wordSize
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for i := 0; i < n; i++ {
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dw[i] = aw[i] ^ bw[i]
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}
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}
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// fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
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// The slice arguments a and b are assumed to be of equal length.
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func xorWords(dst, a, b []byte) {
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if supportsUnaligned {
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fastXORWords(dst, a, b)
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} else {
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safeXORBytes(dst, a, b, len(b))
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}
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}
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